The value of measurement of the atrioventricular (AV) Wenckebach point at rest as a predictor of progression to AV block was investigated prospectively. Twenty-four patients with sinoatrial disease without evidence of conduction disturbance on 12-lead ECG or 24-hour ambulatory monitoring were paced with Medtronic Activitrax II, Medtronic Legend, or Telectronics Meta MV systems in AAI or AAIR modes. Patients were monitored for symptoms and evidence of AV block on 24-hour tapes. The mean age of the patients was 67 years (range: 42-88). There were 11 males and 13 females. The mean follow-up time was 10.7 +/- 5 months. Four patients required revision of pacing system as a result of development of AV block during follow-up. One other patient manifested intermittent second-degree AV block and remains in AAI. The AV Wenckebach points measured at 1 month postimplantation in the four patients who developed AV block requiring revision of system were 140, 125, 165, and 60 (mean 123 +/- 4). The mean AV Wenckebach point at first assessment in the remaining 20 patients was 153 +/- 24. The mean age of those requiring revision of system was 71 +/- 7 compared with 67 +/- 14 in those who did not. In this small series the frequency of development of significant AV block was 17%. This is markedly higher than in other recently reported series. The study demonstrates that an AV Wenckebach point above 120/min does not confer immunity from progression to AV block.